1. Field of the Invention
The present invention relates to the field of power controllers, and particularly to a digital control type power controller for a switching power supply.
2. Description of Related Art
The recent spread of cellular phones has been increasing a demand for lower power consumption of application processors included in cellular phones. To fulfill such demand for lower power consumption, required is a high-precision power control circuit capable of high-speed start-up and of suppressing variation in output voltage. To implement PID (Proportional-Integral-Derivative) control or highly-accurate follow-up control of variation in such a highly-precise power control circuit, a complex arithmetic expression is required, and thereby the number of operation times is increased.
Such being the case, the demand is difficult to fulfill by using an analog circuit; hence, to fulfill the demand, the use of a digital circuit is needed. However, operations by the digital circuit increase power consumption, and consequently shorten the battery life of the cellular phone or the like.
FIG. 9 is a diagram showing an example of a configuration of a power controller of a related art. Referring to FIG. 9, a power controller 201 includes: a switching transistor 2 which receives input voltage 1 supplied to the power controller 201; an inductor 3 connected, at one end, to an output of the switching transistor 2; a capacitor 4 connected between the other end of the inductor 3 and the ground; a power control circuit 70 which receives output voltage 13 and controls the output voltage 13 so that the output voltage 13 would be equal to a target value, the output voltage 13 obtained by smoothing output voltage from the switching transistor 2 by the inductor 3 and the capacitor 4; and a switching transistor driver 12 which performs on/off control on the switching transistor 2 upon receipt of a PWM signal 11 outputted from the power control circuit 70.
The power control circuit 70 includes: an A/D (analog/digital) converter 6 which samples the output voltage 13 (analog voltage) and converts the output voltage 13 into a digital signal; an arithmetic circuit (subtracter) 8 which calculates the difference between a voltage reference (digital value) 7, setting the target value of the output voltage 13, and the output signal of the A/D converter 6; a DSP (Digital Signal Processor) 20 constituting a PID controller which calculates by using, as an input, difference voltage (digital value) 42 outputted from the arithmetic circuit 8, on the basis of three controls, proportional control (P), integral control (I) and derivative control (D); a PWM (Pulse Width Modulation) generation circuit 10 which generates a switching pulse (PWM signal) 11 on the basis of the calculation result obtained by the DSP 20. The PWM signal 11 is inputted to the switching transistor driver 12.
In the power control circuit used in the power controller 201 in FIG. 9, a power switching controller is implemented as a digital signal processing system using digital PWM, the system including the A/D converter 6 and the DSP 20.
The on/off control is performed on the switching transistor 2 by converting the output voltage 13 (or output current) into a digital signal by the A/D converter 6, performing feedback, making comparison with the voltage reference value (digital value) 7, executing an algorithm to cancel the difference at the DSP 20, and then outputting the PWM signal 11.
Although an analog power supply generates the PWM signal 11 in an analog manner by using a comparator and a sawtooth wave (triangular wave), a digital power supply generates the PWM signal 11 in a digital manner as shown in FIG. 9.
If the digital value obtained by A/D conversion of the output voltage 13 is larger than the voltage reference (digital value) 7, then control is performed to turn off the switching transistor 2, while, if the digital value is smaller than the voltage reference 7, then control is performed to turn on the switching transistor 2. In this manner, the output voltage 13 is controlled to be constant and is then supplied.
For example, as a digital type power control, Patent Document 1 discloses a configuration including: an A/D converter which performs A/D conversion on output voltage from a power line; and a voltage monitoring circuit which generates a control signal on the basis of a result of comparison between an output of the A/D converter and a value of a control register (holding a value corresponding to an operation mode), supplies the control signal to a step-down circuit, and adjusts the output voltage to an optimal output voltage. In addition, as a digital type power control circuit, the descriptions of Non-patent Document 1 and the like can also be used as references.    [Patent Document 1] Japanese Patent Application Publication No. 2003-284322    [Non-patent Document 1] Keigo Kimura, Ibuki Mori, Yoshihisa Yamada, Haruo Kobayashi, Yasunori Kobori, Kazuya Shimizu, Masashi Kono, Hao San “High-Resolution DPWM Generator for Digitally Controlled DC-DC Converters,” The 20th Workshop on Circuits and Systems in Karuizawa, Apr. 23-24, 2007, Citation: pp. 551-552 and FIG. 1